A Graphical Method for Predicting Surfactant and Protein Adsorption Properties

Theory and practice of a new method of correlating surface energetics (wettability) with surfactant and protein adsorption properties from aqueous solution are introduced. The method quantified adsorption through an index comprised of solid-liquid interfacial tensions measured by contact-angle goniometry. This adsorption index was shown to have nomographic utility when plotted against surface wettability. The resulting graphical construction, termed an adsorption map, had physical boundaries that restricted index data to regions defining interfacial excess (adsorption) or depletion, allowing straightforward prediction of the surface wettability required to enhance or defeat adsorption. Adsorption mapping was shown to be applicable to both single- and multiple-component solutions of known or unknown chemical composition. Theoretical predictions were tested against results obtained with nonionic, anionic, and cationic surfactants of known chemical composition that exhibited different mechanisms of adsorption. Glass coverslips with or without a hydrophobic silane coating and oxidized polystyrene plaques served as test substrata. Adsorption mapping results were corroborated by the surface thermodynamic method of measuring adsorption by concentration-dependent contact angles. The surface spectroscopies ESCA and SSIMS were applied to obtain direct chemical evidence of strongly-adsorbed cationic surfactant to oxidized surfaces to provide additional confirmation of adsorption mapping results. A simple mathematical model of adsorption was presented that allowed interpretation of linear-like trends observed in the mapping of surfactants, purified proteins (human serum albumin and bovine γ-globulin), and heterogenous mixtures of blood proteins (fetal bovine serum and porcine plasma).